Method for producing solution of polymers functionalized by acid groups by microwave radiation

ABSTRACT

Process for using microwave irradiation to prepare solutions of polymers functionalized with acid groups The invention relates to a process for preparing aqueous, hydrous and anhydrous solutions of polymers functionalized with acid groups, which comprises using microwave radiation to supply the heat required to prepare the solution. The solutions are suitable as a starting material for producing gas diffusion electrodes, fuel cells and polymer-electrolyte-stabilized platinum nanoparticles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Process for using microwave irradiation to prepare solutions of polymersfunctionalized with acid groups.

2. Description of the Prior Art

The present invention relates to a process for using microwave radiationto prepare solutions of polymers functionalized with acid groups, to aprocess for preparing the solid, soluble or insoluble polymers, and alsoto the use of the solutions.

Many high-performance polymers, such as polyether ketones, partiallyfluorinated or perfluorinated polymers and polyphenylene sulfides, arehighly insoluble. While this insolubility is precisely what is requiredfor many applications, it does, however, make the processing of thesepolymers very considerably more difficult, or in extreme casesimpossible.

The derivatives of these polymers functionalized with acid groups, suchas —SO₃H, —B(OH)₂, —CO₂H and —PO₃H are, in contrast and depending ontheir degree of functionalization, soluble in solvents such asdimethylformamide, dimethyl sulfoxide, dimethyl-acetamide orN-methylpyrrolidone. With particularly high degrees of functionalizationthese polymers may also become water-soluble.

There is great demand for aqueous solutions of polymers of this type,functionalized with acid groups. Examples of the advantages ofdispensing with organic solvents are cost reduction, environmentalprotection factors and factors relating to workforce health. It is alsodesirable, and particularly in the field of preparation of noble-metalcatalyst materials, to avoid, or at least minimize, the use of solventscontaining hetero-atoms, in particular containing chlorine, sulfur ornitrogen, since these can act as catalyst poisons. This is the reasonfor, for example, interest in aqueous preparations of proton-conductingpolymers for producing gas diffusion electrodes for fuel cells orelectrolysis units.

U.S. Pat. No. 5,453,161 discloses the preparation of polyimides derivedfrom benzophenone-3,3′,4,4′-tetracarboxylic acid, where the reactionmixture is heated by microwave radiation. The product is not reported tobe water-soluble.

JP-05 310 907 discloses a process for removing methylene chloride frompolymers by using microwave radiation. There is no report that thesolubility of the polymers is higher after the microwave irradiationthan is the case with traditional processes for introducing heat.

Against the background of the prior art, the object was to develop aprocess by which polymers carrying acid groups can be prepared in a formwhich gives rise to greater solubility in water and in organic solvents.

SUMMARY OF THE INVENTION

Surprisingly, it has been found that polymers carrying acid groups aremore soluble in water and in organic solvents when exposed to microwaveradiation than when exposed to heat and pressure.

The invention provides a process for preparing aqueous, hydrous andanhydrous solutions of polymers functionalized with acid groups, whichcomprises using microwave radiation to supply the heat required toprepare the solution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Even polymers whose low degree of functionalization gives them no, oronly very slight, solubility, even on heating under pressure(temperature up to 175° C., pressure up to 4 bar) can be dissolved bythis process. The acid groups with which the polymers have beenfunctionalized are preferably sulfonic acid, phosphoric acid, carboxyland/or boric acid groups.

Possible solvents are especially water, dimethyl-acetamide,N-methylpyrrolidone, dimethyl-formamide, dimethyl sulfoxide, alcoholssuch as isopropanol, and also mixtures of two or more of thesesubstances.

The polymers preferably used are polyether ketones, polyphenylenesulfides, partially fluorinated or perfluorinated aliphatic polymers orpolyether sulfones, in particular those with an ion-exchange capacity(IEC) of from 0.5 to 2 mmol of acid function per g of polymer.Particular preference is given to the use of polymers of the formula 1

which are marketed by DuPont with the tradename ®Nafion. Thecoefficients are: Z≧1, m=from 5 to 14, and n=100.

Unlike when heated without microwave irradiation, it has been found thatespecially sulfonated polyether ketones (PEKs), polyether ether ketones(PEEKs) and polyether ether ketone ketones (PEEKKs) can be dissolvedwith a significantly lower degree of sulfonation with microwaveirradiation. No detectable molecular weight degradation takes place whenthe novel process is carried out. The solution result achieved here,under relatively mild conditions (low temperature and pressure), is atleast comparable and in fact mostly significantly better than whenheating without microwave irradiation. It is clear that the microwaveradiation itself, and not the heat released by the microwave radiation,is responsible here for the good solubility performance of the polymersunder these conditions.

In addition, polymers whose low degree of functionalization gives themno, or only slight, solubility in a non-aqueous solvent withoutmicrowave irradiation when heating, even under pressure, can bedissolved using microwave irradiation in N-methylpyrrolidone,dimethylacetamide, dimethyl-formamide, org. sulfoxide, such as sulfolan,or in dimethyl sulfoxide. This makes it possible for the first time forpolymers with a low degree of functionalization, i.e. with a smallproportion of derivatized repeat units, to be processed from solution.For example, it is possible to process sulfonated polyether ketones witha degree of sulfonation of ≦35% from NMP solution.

The invention also provides a process for obtaining the solid polymersfrom the microwave-irradiated solutions. After the novel process hasbeen carried out, these polymers are insoluble in the solvent from whichthey were obtained.

The solutions of the polymers in water may be concentrated byevaporation to dryness. The solid polymer regained in this way may bedissolved in water by heating without microwave irradiation, or annealedto become water-insoluble, and from this condition may once again beconverted into a water-soluble condition by microwave irradiation. Theadvantage of this surprising property is that the polymers can betransported in a soluble form without solvents, then can be dissolvedwithout microwave irradiation and, after processing and after removingthe solvent by evaporation, can be annealed to become insoluble.

No degradation of molecular weight takes place here during the microwaveirradiation, as can be shown with the aid of gel permeationchromatography (GPC) or thermal field flow fractionation (TFFF).Dissolved and undissolved fractions also show no differences inmolecular weights and the degree of sulfonation which they show isuniform. In the course of dissolving, therefore, no extraction of morehighly sulfonated or low-molecular-weight compounds takes place.

The solutions prepared by the novel process are suitable, for example,for producing gas diffusion electrodes, fuel cells andpolymer-electrolyte-stabilized platinum nanoparticles.

EXAMPLES

In all of these solutions experiments with microwave radiation themicrowave apparatus used was the CET model MDS 2000.For safety reasons,the apparatus is operated at only 50% of the nominal rating, with apressure limit of 3.99 bar and with a temperature limit of 1750° C. Thesample vessels used were Teflon autoclaves with screw fittings. Beforethe microwave radiation is switched on, the sample vessels are flushedwith nitrogen.

Example 1

Sulfonated PEEK, dissolved in water using microwave irradiation.

47.5 g of water and 2.5 g of ground, sulfonated PEEK with a degree ofsulfonation of 51% are placed in a microwave autoclave and flushed withnitrogen for 4 minutes. The microwave apparatus is then switched on for5 minutes. After the gauge pressure has fallen to 0.1 bar, the autoclaveis opened and the resultant solution is centrifuged at 4500 rpm for 30min. The resultant clear solution is concentrated by evaporation.Determining the dry weight of the solution shows that 98.3% of thepolymer has dissolved. The molar mass of the polymer is determined byGPC (system: Waters, temperature 650° C., polystyrene calibration,solvent: NMP, with the addition of 0.05% of lithium chloride). Thenumber-average molar mass and the weight-average molar mass here are,respectively, 65,000+/−3000 g/mol and 160,000+/−8000 g/mol, for thestarting polymer, for the solution of the polymer and also for theundissolved polymer.

Example 2

Sulfonated PEEKK, dissolved in water using microwave irradiation.

47.5 g of water and 2.5 g of ground, sulfonated PEEK with a degree ofsulfonation of 65% are placed in a microwave autoclave. The autoclave isclosed and flushed with nitrogen for 5 minutes. The microwave apparatusis switched on for 10 minutes. After cooling, the solution iscentrifuged at 4500 rpm for 30 min. The water-soluble fraction of thepolymer is 1.989 g (79%). The undissolved residue is 0.461 g (19%). Themolar mass of the polymer is determined by GPC (system: Waters,temperature 650° C., polystyrene calibration, solvent: NMP, with theaddition of 0.05% of lithium chloride). The number-average molar massand the weight-average molar mass here are, respectively, 55,000 +/−3000g/mol and 130,000 +/−8000 g/mol, for the starting polymer, for thesolution of the polymer and also for the undissolved polymer.

Comparative Example 3

Sulfonated PEEKK is dissolved in water without microwave irradiation.

1 g of sulfonated, ground PEEKK with a degree of sulfonation of 65% ischarged to a glass autoclave together with 19 g of distilled water. Anoil bath is used to heat this for 40 min to a temperature of 165° C.(internal pressure 3.5 bar), followed by cooling. This gives a brownishgel. The supernatant liquid is slightly cloudy and comprises only about0.05 g of polymer, corresponding to 5%.

Example 4

Sulfonated PEEK, dissolved in N-methylpyrrolidone using microwaveirradiation.

3 g of sulfonated, ground PEEK with a degree of sulfonation of 33% arecharged to a microwave autoclave together with 57 g ofN-methylpyrrolidone. The microwave autoclave is flushed with nitrogenfor 5 min. The microwave apparatus is then switched on for 5 minutes.After cooling, this gives a clear, pale yellow solution with a brownishsediment. The soluble fraction is 84% (2.53 g).

What is claimed is:
 1. A process for preparing an aqueous, hydrous oranhydrous solution of a polymer functionalized with one or more acidgroups, which comprises heating the polymer in a solvent by exposing itto microwave radiation to form the polymer solution.
 2. The process asclaimed in claim 1, wherein the one or more acid groups are sulfonicacid, phosphoric acid, carboxyl and/or boric acid groups.
 3. The processas claimed in claim 1, wherein the solvent is water, dimethylacetamide,N-methylpyrrolidone, dimethylformamide, an organic sulfoxide, analcohol, or a mixture thereof.
 4. The process as claimed in claim 3,wherein the alcohol is isopropanol.
 5. The process as claimed in claim3, wherein the organic sulfixide is sulfolan or dimethyl sulfoxide. 6.The process as claimed in claim 1, wherein the polymer is a polyetherketone, a polyphenylene sulfide, a polyether sulfone or a compound ofthe formula 1:

where Z≧1, m=from 5 to 14 and n=100.
 7. The process as claimed in claim6, wherein the polyether ketone is a sulfonated polyether ketone, apolyether ether ketone or a polyether ether ketone ketone.
 8. Theprocess as claimed in claim 1, wherein the polymer has an ion-exchangecapacity of from 0.5 to 2 mmol of acid functionality per g of thepolymer.
 9. A process for preparing a polymeric solid from the polymersolution of the process as claimed in claim 1, comprising evaporatingoff the solvent from the polymer solution to form the polymeric solid,wherein the polymeric solid is soluble in the solvent from which it wasobtained.
 10. The process as claimed in claim 9, further comprisingannealing the polymeric solid such that it is insoluble in the solventwhich was used with the microwave irradiation to prepare the polymersolution.
 11. The process as claimed in claim 1, wherein the polymer isa partially fluorinated or perfluorinated aliphatic polymer.
 12. Theprocess as claimed in claim 1, wherein the polymer is a sulfonatedpolyether ether ketone and the solvent is water.
 13. The process asclaimed in claim 1, wherein the polymer is a sulfonated polyether etherketone ketone and the solvent is water.
 14. The process as claimed inclaim 1, wherein the polymer is a sulfonated polyether ether ketone andthe solvent is N-methyl pyrrolidone.
 15. The process as claimed in claim1, wherein the polymer is a sulfonated polyether ketone having a degreeof sulfonation of ≦35% and the solvent is N-methyl pyrrolidone.
 16. Apolymeric solid prepared by the process as claimed in claim
 9. 17. Apolymeric solid prepared by the process as claimed in claim
 10. 18. Aprocess for preparing a gas diffusion electrode, a fuel cell or apolymer-electrolyte stabilized, platinum nanoparticle, comprisingconverting the polymer solution of the process as claimed in claim 1into the electrode, the fuel cell or the nanoparticle.
 19. A gasdiffusion electrode, a fuel cell or a polymer-electrolyte stabilized,platinum nonoparticle prepared by the process as claimed in claim 18.